Fluid catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke is deposited on the catalyst. A high temperature regeneration operation within a regeneration zone combusts coke from the catalyst. Coke-containing catalyst, referred to herein as coked catalyst or spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and the regeneration zone.
In the regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Flue gas formed by burning the coke in the regenerator is treated for removal of particulates and conversion of carbon monoxide, after which the flue gas may be normally discharged into the atmosphere. Conventional regenerators typically include a vessel having a coked catalyst inlet, a regenerated catalyst outlet and a combustion gas distributor for supplying air or other oxygen containing gas to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the flue gas before the gas exits the regenerator vessel.
There are several types of catalyst regenerators in use today. A conventional bubbling bed regenerator typically has just one chamber in which air is bubbled through a dense catalyst bed. Coked catalyst is added, and regenerated catalyst is withdrawn from the same dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed. Two-stage bubbling beds have two chambers. Coked catalyst is added to a dense bed in a first chamber and is partially regenerated with air. The partially regenerated catalyst is transported to a dense bed in a second chamber and completely regenerated with air. The completely regenerated catalyst is withdrawn from the second chamber with usually less than 0.1 wt % residual coke.
A combustor-style regenerator or high efficiency regenerator has a lower chamber called a combustor that burns nearly all the coke to CO2 with little or no CO promoter and typically with low excess oxygen. A portion of the hot regenerated catalyst from the upper regenerator is recirculated to the lower combustor to heat the incoming spent catalyst and to control the combustor catalyst density and temperature for optimum coke combustion rate. As the catalyst and flue gas mixture enters an upper, narrower section of the combustor, the upward velocity is further increased and the two-phase mixture exits through a disengager into an upper chamber. The upper chamber separates the catalyst from the flue gas in the disengager and cyclones and returns the catalyst to a dense catalyst bed which supplies hot regenerated catalyst to both the riser reactor and the lower combustor chamber.
Therefore, there is a need for improved methods for combustion of coke from catalyst in vessels having a shorter elevation.